Companion Dwarf Galaxy Almost Invisible

A team of astronomers has discovered the least luminous, most dark matter-filled galaxy known to exist. The Segue 1 galaxy is one of about two dozen small satellite galaxies orbiting our own Milky Way. This is a very faint galaxy, a billion times less bright than the Milky Way. But despite its small number of visible stars, Segue 1 is nearly a thousand times more massive than it appears, meaning most of its mass must come from dark matter. “Segue 1 is the most extreme example of a galaxy that contains only a few hundred stars, yet has a relatively large mass,” said Marla Geha, an assistant professor of astronomy at Yale and lead author on a paper about Segue 1.

Geha and her colleagues have observed about half of the dwarf satellite galaxies that orbit the Milky Way. These objects are so faint and contain so few stars that at first they were thought to be globular clusters – tightly bound star clusters that also orbit our host galaxy. But by analyzing the light coming from the objects using the Keck telescope in Hawaii, the researchers determined these objects are actually galaxies, but just very faint.

Looking only at the light emitted by these ultra-faint galaxies, Geha and her colleagues expected them to have correspondingly low masses. Instead, they discovered that they are between 100 and 1000 times more massive than they appear. Invisible dark matter, she said, must account for the difference.

Although dark matter doesn’t emit or absorb light, scientists can measure its gravitational effect on ordinary matter and believe it makes up about 85 percent of the total mass in the universe. Finding ultra-faint galaxies like Segue 1, which is so rife with dark matter, provides clues as to how galaxies form and evolve, especially at the smallest scales.

“These dwarf galaxies tell us a great deal about galaxy formation,” Geha said. “For example, different theories about how galaxies form predict different numbers of dwarf galaxies versus large galaxies. So just comparing numbers is significant.”

It’s only recently that astronomers have discovered just how prevalent these dwarf satellite galaxies are, thanks to projects like the Sloan Digital Sky Survey, which imaged large areas of the nighttime sky in greater detail than ever before. In the past two years alone, the number of known dwarf galaxies orbiting the Milky Way has doubled from the dozen or so brightest that were discovered during the first half of the twentieth century.

Geha predicts astronomers will find even more as they continue to sift through new data. “The galaxies I now consider bright used to be the least luminous ones we knew about,” she said. “It’s a totally new regime. This is a story that’s just unfolding.”

Source: Yale University

Do All Galaxies Have Tentacles?

This Hubble Space Telescope image of two spiral galaxies shows an interesting feature on the smaller galaxy. Silhouetted in front of the larger background galaxy is a small galaxy, and tentacles of dust can be seen extending beyond the small galaxy’s disk of starlight. These dark, dusty structures appear to be devoid of stars, almost like barren branches. They are rarely so visible in a galaxy because there is usually nothing behind them but darkness. But here, with the backdrop of the larger galaxy they are illuminated. Astronomers have never seen dust this far beyond the visible edge of a galaxy, and they don’t know if these dusty structures are common features in galaxies.

The background galaxy is 780 million light-years away, but the distance between the two galaxies has not yet been calculated. Astronomers think the two are relatively close, but not close enough to actually interact. The background galaxy is about the size of the Milky Way Galaxy and is about 10 times larger than the foreground galaxy. Understanding a galaxy’s color and how dust affects and dims that color are crucial to measuring a galaxy’s true brightness. By knowing the true brightness, astronomers can calculate the galaxy’s distance from Earth.

Most of the stars speckled across this image belong to the nearby spiral galaxy NGC 253, which is out of view to the right. Astronomers used Hubble’s Advanced Camera for Surveys to snap images of NGC 253 when they spied the two galaxies in the background. From ground-based telescopes, the two galaxies look like a single blob. But the Advanced Camera’s sharp “eye” distinguished the blob as two galaxies, cataloged as 2MASX J00482185-2507365. The images were taken on Sept. 19, 2006.

Source: Hubblesite

Our Sun May Have Migrated Over Time

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When you stir cream in your coffee or tea, does the swirl stay the same or does it change as it spins in your cup? As galaxies form and swirl, the motions and eddies may actually cause stars to move within the galaxy. A long-standing scientific belief holds that stars tend to hang out in the same general part of a galaxy where they originally formed. But some astrophysicists have recently questioned whether that is true, and now new simulations show that, at least in galaxies similar to our own Milky Way, stars such as the sun can migrate great distances. If this is true, it could change the entire notion that there are parts of galaxies – so-called habitable zones – that are more conducive to supporting life than other areas.

“Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy’s disk but not others,” said Rok RoÅ¡kar, a doctoral student in astronomy at the University of Washington. “If stars migrate, then that zone can’t be a stationary place.”

RoÅ¡kar is lead author of a paper describing the findings from the simulations, published in the Sept. 10 edition of the Astrophysical Journal Letters. If the idea of habitable zone doesn’t hold up, it would change scientists’ understanding of just where, and how, life could evolve in a galaxy, he said.

Using more than 100,000 hours of computer time on a UW computer cluster and a supercomputer at the University of Texas, the scientists ran simulations of the formation and evolution of a galaxy disk from material that had swirled together 4 billion years after the big bang. Watch a simulation video.

The simulations begin with conditions about 9 billion years ago, after material for the disk of our galaxy had largely come together but the actual disk formation had not yet started. The scientists set basic parameters to mimic the development of the Milky Way to that point, but then let the simulated galaxy evolve on its own.

If a star, during its orbit around the center of the galaxy, is intercepted by a spiral arm of the galaxy, scientists previously assumed the star’s orbit would become more erratic in the same way that a car’s wheel might become wobbly after it hits a pothole.

However, in the new simulations the orbits of some stars might get larger or smaller but still remain very circular after hitting the massive spiral wave. Our sun has a nearly circular orbit, so the findings mean that when it formed 4.59 billion years ago (about 50 million years before the Earth), it could have been either nearer to or farther from the center of the galaxy, rather than halfway toward the outer edge where it is now.

Migrating stars also help explain a long-standing problem in the chemical mix of stars in the neighborhood of our solar system, which has long been known to be more mixed and diluted than would be expected if stars spent their entire lives where they were born. By bringing in stars from very different starting locations, the sun’s neighborhood has become a more diverse and interesting place, the researchers said.

The findings are based on a few runs of the simulations, but the scientists plan to run a range of simulations with varying physical properties to generate different kinds of galactic disks, and then determine whether stars show similar ability to migrate large distances within different types of disk galaxies.

Source: University of Washington

Pretty Picture of the Day: M83

What a great way to start the day, with a gorgeous image like this one of the galaxy Messier 83, adorned with what looks like rubies on the spiral arms. This shot was captured by the Wide Field Imager at ESO’s La Silla Observatory, located high in the dry desert mountains of the Chilean Atacama Desert. Messier 83 lies roughly 15 million light-years away towards the southern constellation of Hydra. To make this image, the WFI stared at M83 for roughly 100 minutes through a series of specialist filters, allowing the faint detail of the galaxy to reveal itself. The brighter stars in the foreground are stars in our own galaxy, and behind M83 the darkness is peppered with the faint smudges of distant galaxies.

M83 stretches over 40,000 light-years, making it roughly 2.5 times smaller than our own Milky Way. However, in some respects, Messier 83 is quite similar to our own galaxy. Both the Milky Way and Messier 83 possess a bar across their galactic nucleus, the dense spherical conglomeration of stars seen at the centre of the galaxies.

The red, ruby like features are in fact huge clouds of glowing hydrogen gas. Ultraviolet radiation from newly born, massive stars is ionizing the gas in these clouds, causing the great regions of hydrogen to glow red. These star forming regions are contrasted dramatically in this image against the ethereal glow of older yellow stars near the galaxy’s central hub. The image also shows the delicate tracery of dark and winding dust streams weaving throughout the arms of the galaxy.

Messier 83 was discovered by the French astronomer Nicolas Louis de Lacaille in the mid 18th century. Decades later it was listed in the famous catalogue of deep sky objects compiled by another French astronomer and famous comet hunter, Charles Messier.

Source: ESO

Clash of Clusters Separates Dark Matter From Ordinary Matter

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A powerful collision of galaxy clusters captured by NASA’s Hubble Space Telescope and Chandra X-ray Observatory provides evidence for dark matter and insight into its properties. Observations of the cluster known as MACS J0025.4-1222 indicate that a titanic collision has separated dark matter from ordinary matter. The images also provide an independent confirmation of a similar effect detected previously in a region called the Bullet Cluster. Like the Bullet Cluster, this newly studied cluster shows a clear separation between dark and ordinary matter.

MACS J0025 formed after an enormously energetic collision between two large clusters. Using visible-light images from Hubble, the team was able to infer the distribution of the total mass — dark and ordinary matter. Hubble was used to map the dark matter (colored in blue) using a technique known as gravitational lensing. The Chandra data enabled the astronomers to accurately map the position of the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (pink).

As the two clusters that formed MACS J0025 (each almost a whopping quadrillion times the mass of the Sun) merged at speeds of millions of miles per hour, the hot gas in the two clusters collided and slowed down, but the dark matter passed right through the smashup. The separation between the material shown in pink and blue therefore provides observational evidence for dark matter and supports the view that dark-matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.

On the Chandra website, there are two animations, one that shows the different views of this cluster viewed by the different observatories, and another depicting how the galaxies may have collided.

Bullet Cluster.  Credit:  NASA/CXC/CfA/STScI
Bullet Cluster. Credit: NASA/CXC/CfA/STScI

These new results show that the Bullet Cluster is not an anomalous case and helps answers questions about how dark matter interacts with itself.

Sources: HubbleSite, Chandra

Cosmic ‘Needle in a Haystack’ Confirms Dark Energy

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A massive cluster of galaxies seen in the distant universe by ESA’s orbiting XMM-Newton x-ray observatory is so big that astronomers believe there can only be a few of them that far away in space and time. “Such massive galaxy clusters are thought to be rare objects in the distant Universe,” said Georg Lamer, Astrophysikalisches Institut in Potsdam, Germany. “They can be used to test cosmological theories. Indeed, the very presence of this cluster confirms the existence of a mysterious component of the Universe called dark energy.” The astronomers compared the rare find to a cosmic ‘needle in a haystack.’

The newly-discovered monster, known by the catalogue number 2XMM J083026+524133, is 7.7 thousand million light-years distant and is estimated to contain as much mass as a thousand large galaxies. Much of it is in the form of 100-million-degree hot gas. The bright blue blob of gas was found during a systematic analysis of catalogued objects as Lamer and his team were looking for patches of X-rays that could either be nearby galaxies of distant clusters of galaxies.

Based on 3,500 observations performed with XMM-Newton’s European Photon Imaging Camera (EPIC) covering about 1% of the entire sky, the catalogue contains more than 190,000 individual X-ray sources. J083026+524133 stood out because it was so bright. While checking visual images from the Sloan Digital Sky Survey, the team could not find any obvious nearby galaxy in that location. So they turned to the Large Binocular Telescope in Arizona and took a deep exposure, which found a cluster of galaxies in that location.

The astronomers were surprised to find the cluster contains a thousand times the mass of our own Milky Way Galaxy.

No one knows what dark energy is, but it is causing the expansion of the Universe to accelerate. This hampers the growth of massive galaxy clusters in more recent times, indicating that they must have formed earlier in the Universe. “The existence of the cluster can only be explained with dark energy,” says Lamer.

Yet he does not expect to find more of them in the XMM-Newton catalogue. “According to the current cosmological theories, we should only expect to find this one cluster in the 1% of sky that we have searched,” says Lamer.

Source: ESA

Hubble Spies Beautiful, Beastly Monster Galaxy

Complete with tentacles, a supermassive black hole and x-ray emitting gas, a monster of a galaxy has been found by NASA’s Hubble Space Telescope, and is helping astronomers answer a long-standing puzzle. The very active galaxy NGC 1275 has giant but beautiful and delicate filaments influenced and shaped by a beastly-strong extragalactic magnetic field. But how the delicate structures such as those found in this galaxy can withstand the hostile, high-energy environment has been a mystery. But researchers say the beauty and the beast co-exist and are dependent on each other for survival.

One of the closest giant elliptical galaxies, NGC 1275 hosts a supermassive black hole. Energetic activity of gas swirling near the black hole blows bubbles of material into the surrounding galaxy cluster. Long gaseous filaments stretch out beyond the galaxy, into the multimillion-degree, X-ray–emitting gas that fills the cluster. Astronomers thought these delicate filaments should have heated up, dispersed, and evaporated by now, or collapsed under their own gravity to form stars.

These filaments are the only visible-light manifestation of the intricate relationship between the central black hole and the surrounding cluster gas. They provide important clues about how giant black holes affect their surrounding environment.

Using Hubble’s view, a team of astronomers led by Andy Fabian from the University of Cambridge, UK, have for the first time resolved individual threads of gas that make up the filaments. The amount of gas contained in a typical thread is around one million times the mass of our own Sun. They are only 200 light-years wide, are often very straight, and extend for up to 20,000 light-years. The filaments are formed when cold gas from the core of the galaxy is dragged out in the wake of the rising bubbles blown by the black hole.

A new study published in the August 21 Nature magazine proposes that magnetic fields hold the charged gas in place and resist the forces that would distort the filaments. This skeletal structure is strong enough to resist gravitational collapse.

“We can see that the magnetic fields are crucial for these complex filaments – both for their survival and for their integrity,” said Fabian.

Similar networks of filaments are found around other more remote central cluster galaxies. However, they cannot be observed with comparable resolution to the view of NGC 1275. In future observations, the team will apply the understanding of NGC 1275 to interpret what they see in other, more distant galaxies.

News Source: Hubble Site

Dark Matter is Missing From Cosmic Voids

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Cosmic voids really are devoid of matter. Astronomers have found that even the pervasive ‘dark matter’ which accounts for about 80% of the mass of the universe is not present in these voids, which are areas of vast emptiness in space that can be tens of millions of light-years across. “Astronomers have wondered for a quarter-century whether these voids were ‘too big’ or ‘too empty’ to be explained by gravity alone,” said University of Chicago researcher Jeremy Tinker, who led the new study using data from the Sloan Digital Sky Survey II (SDSS-II). “Our analysis shows that the voids in these surveys are exactly as big and as empty as predicted by the ‘standard’ theory of the universe.”

The largest 3-dimensional maps of the universe show that galaxies lie in filamentary superclusters interlaced by cosmic voids that contain few or no bright galaxies. Researchers using SDSS-II and the
Two-Degree Field Galaxy Redshift Survey (2dFGRS) have concluded that these voids are also missing the “halos” of invisible dark matter that bright galaxies reside in.

A central element of the standard cosmological theory is cold dark matter, which exerts gravity but does not emit light. Dark matter is smoothly distributed in the early universe, but over time gravity pulls it into filaments and clumps and empties out the spaces between them. Galaxies form when hydrogen and helium gas falls into collapsed dark matter clumps, referred to as “halos,” where it can form luminous stars.

But astronomers were not sure if the areas that are devoid of galaxies were also devoid of dark matter, or if the dark matter was there, but for some reason stars just didn’t form in these voids.
The research team used bright galaxies to trace the structure of dark matter and compared it with computer simulations to predict the number and sizes of voids.
Princeton University graduate student Charlie Conroy measured the sizes of voids in the SDSS-II maps. “When we used galaxies brighter than the Milky Way to trace structure, the biggest empty voids we found were about 75 million light years across,” said Conroy. “And the predictions from the simulations were bang-on.”

The sizes of voids are ultimately set, Conroy explained, by the small variations in the primordial distribution of dark matter, and by the amount of time that gravity has had to grow these small variationsinto large structures.

The agreement between the simulations and the measurements holds for both red (old) and blue (new) galaxies, said Tinker. “Halos of a given mass seem to form similar galaxies, both in numbers of stars and in the ages of those stars, regardless of where the halos live.”

Tinker presented his findings today at an international symposium in Chicago, titled “The Sloan Digital Sky Survey: Asteroids to Cosmology.” A paper detailing the analysis will appear in the September 1 edition of The Astrophysical Journal, with the title “Void Statistics in Large Galaxy Redshift Surveys: Does Halo Occupation of Field Galaxies Depend on Environment?”

News Source: SDSS and The Ohio State University

Milky Way Creates a Mess by Stealing Stars from Nearby Galaxies

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The halo of stars that envelops the outer Milky Way galaxy is like a “jumble of pasta” said one researcher, describing criss-crossed patterns of stellar streams revealed in new data from the Sloan Digital Sky Survey (SDSS). These stars appear to have been ripped away from the dwarf galaxies that are companions to our own galaxy, creating messy, spaghetti-like streams of stars in the outer edge of the Milky Way. The SEGUE (Sloan Extension for Galactic Understanding and Exploration) of the Sloan Survey is mapping the structure and stellar makeup of the Milky Way Galaxy and has found numerous new small streams of stars mixed and tangled among larger streams that had been mapped out over the last decade. It appears the Milky Way’s thievery is creating quite a mess.

While the center of galaxy is quite orderly, the outer Milky Way is a cluttered mess. Kathryn Johnston from Columbia University explained how dwarf galaxies that pass close to the Milky Way can be stretched by gravitational tides into spaghetti-like strands, which wind around the Galaxy as stars trace out the same orbital paths at different rates.

“In the center of the Galaxy, these stellar strands crowd together and you just see a smooth mix of stars,” said Johnston. “But as you look further away you can start to pick out individual strands, as well as features more akin to pasta shells that come from dwarfs that were on more elongated orbits.” Johnston described the new smaller strands recently detected as “angel hair” that came from smaller dwarf or ones that were destroyed longer ago.

Heidi Newberg of Rensselaer Polytechnic Institute and her thesis student Nathan Cole have been trying to follow some of the larger strands as they weave across the sky. “It’s a big challenge to piece things together,” said Cole, “because the stream from one dwarf galaxy can wrap around the Galaxy and pass through streams of stars ripped from other dwarf galaxies.”

Toward the constellation Virgo, where SDSS images revealed an excess of stars covering a huge area of sky, there are at least two superposed structures, and possibly three or more. The SEGUE velocity measurements can separate systems that overlap in sky maps, Newberg explained. “Part of what we see toward Virgo is a tidal arm of the Sagittarius dwarf galaxy, whose main body lies on the opposite side of the Milky Way, but we don’t know the origin of the other structures. There really aren’t enough pasta varieties to describe all the structures we find.”

“The SDSS has taught us a huge amount about the Milky Way and its neighbors,” said Johnston. “But we’re still just beginning to map the Galaxy in a comprehensive way, and there’s a trove of discoveries out there for the next generation of surveys, including the two new Milky Way surveys that will be carried out in SDSS-III,” the next set of surveys slated for Sloan.

Original News Source: SSDS press release, The Ohio State University

Two Galaxies Walk Into a Bar…

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Two galaxies walk into a bar. The young, regular spiral galaxy and the mature, barred spiral both order a drink. But the bartender only gives a drink to the barred spiral galaxy. The regular spiral galaxy says, “Hey, why didn’t I get my drink?” The bartender replies, “You’re too young, plus we don’t serve your type.”

Extremely lame joke, I know. But now that I have your attention, one of the latest studies conducted by the Hubble Space Telescope show that barred spiral galaxies were less plentiful 7 billion years ago than they are today. This confirms the idea that bars are a sign of galaxies getting older and reaching full maturity; they are no longer in their “formative years.” Using Hubble’s Advanced Camera for Surveys, astronomers say this study of the history of bar formation provides clues to understanding when and how spiral galaxies form and evolve over time.

And if anyone can come up with a better “two galaxies walk into a bar” joke, post it in the comments below. The winner gets a free subscription to Universe Today.

Hubble looked at more than 2,000 spiral galaxies in the Cosmic Evolution Survey (COSMOS). A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts.

Bars have been forming steadily over the last 7 billion years, more than tripling in number. “The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation,” said Sheth. “They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today.”

The findings have important implications for galaxy evolution. “We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time,” he said.

Artist's illustration of the Milky Way.  Credit;  NASA
Artist's illustration of the Milky Way. Credit; NASA

Our own Milky Way Galaxy was recently determined to have a central bar. Our galaxy is another massive barred spiral, and its central bar probably formed somewhat early, like the bars in other large galaxies in the Hubble survey. “Understanding how bars formed in the most distant galaxies will eventually shed light on how it occurred here, in our own backyard,” Sheth said.

COSMOS covers an area of sky nine times larger than the full Moon, surveying 10 times more spiral galaxies than previous observations. In support of the Hubble galaxy images, the team derived distances to the galaxies in the COSMOS field using data from Hubble and an assortment of ground-based telescopes.

Astronomers believe bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. “The tiny elongations in the stars’ orbits grow and they get locked into place, making a bar,” explained team member Bruce Elmegreen of IBM’s research Division in Yorktown Heights, N.Y. “The bar becomes even stronger as it locks more and more of these elongated orbits into place. Eventually a high fraction of the stars in the galaxy’s inner region join the bar.”

Bars are perhaps one of the most important catalysts for changing a galaxy. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes.

“The formation of a bar may be the final important act in the evolution of a spiral galaxy,” Sheth said. “Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars.”

Yes, there’s always lots of action in bars. Especially when two galaxies walk in.

Original News Source: HubbleSite